1. Field of the Invention
The invention lies in the field of chambered high-voltage transformers intended for powering high-voltage electrodes of cathode-ray tubes, such as those used in television receivers or monitors. It relates more particularly to a step-up coil of such a transformer, and the transformer equipped with this coil.
2. Description of the Prior Art
From the technological standpoint, high-voltage transformers may be divided into two major families, chambered transformers and layered transformers. The transformers of these two families comprise a ferromagnetic circuit and primary and secondary windings coiled around at least part of the magnetic circuit. The secondary windings comprise two types of windings, secondary windings which serve to produce auxiliary voltages of for example 5, 12 or 30 volts and windings serving to produce the high voltages required for the operation of the cathode-ray tube, for example the focusing voltage of the order of 7 to 10 kilovolts and the anode voltage of the order of 30 kilovolts. These latter windings are commonly referred to as tertiary windings or else step-up windings. In layered transformers, the step-up windings are mounted around part of the magnetic circuit, in concentric coaxial layers situated one above another in a radial direction with respect to the axis of the magnetic circuit. The various layers of windings are galvanically insulated from one another by layers of a flexible insulating material installed before winding the following layer. In chambered transformers, the step-up windings are galvanically insulated from one another through the fact that they are housed respectively in chambers separated by insulating partitions. These chambers are distributed along an axial line of the magnetic circuit. The transformer according to the invention lies in this latter category, that of chambered transformers. These transformers are already widely known and have been described in the prior art.
Chambered transformers have an advantage over layered technology in so far as the cost of construction is lower, in particular because it is possible to simultaneously coil the windings of several chambers. Moreover, the interruptions required for laying an insulant, for example of the terphane type, between layers are avoided. On the other hand, they exhibit greater so-called "ringing" stray voltages. These oscillations produce perturbations to the image on the screens of cathode-ray tubes. These perturbations of the image are unacceptable on top-range television sets, monitors or televisions with a high definition image. It has been noted that these image perturbations were nonexistent or at least much reduced with layer-technology transformers. The inventors think that this difference stems from what they refer to as inactivation of the inter-layer capacitances. The various inter-layer capacitances is energized at each of their two ends by identical voltage pulses. The alternating variation in voltage across the terminals of these capacitances is therefore zero. The inter-layer stray capacitances not excited. Moreover, these layer-technology transformers benefit from the perfect coupling between the primary winding and each layer of the step-up winding. Moreover, the insertion between the earth and the first section of the step-up coil (first layer) of a dipole consisting of a resistor in parallel with an inductor helps to expunge any residual overoscillation almost completely. The inventors think that for these reasons a voltage devoid of ringing and capable after rectification of delivering a very stable DC level when the screen scanning frequency or the luminance of the image, which determines the beam current, varies is obtained at the end of any intermediate layer chosen to deliver, for example, the focusing voltage. Tracking of focusing is then said to be good. In the chambered technology, the inter-chamber capacitances are activated on account of the fact that the instantaneous voltages present on the windings of two consecutive chambers are different. This results in the generation of stray voltages due to the chargings and dischargings of these capacitances.